texts: Fact Check: How bright will the Perseids be in 2017?

texts:
A friend sent me an email which appears to be an extract from this Cambridge News article:

A meteor shower astronomers have labelled 'a once in a lifetime opportunity' will take over the sky in August. The upcoming Perseid meteor shower will be the brightest in human history so far, Physics Astronomy reported.
Anyone in the Northern Hemisphere should be able to view the shower on August 12. It is predicted to be so bright that stargazers should be able to catch a glimpse even during the day.
Astronomers believe that a shower this visible won't grace the sky for another 96 years. The Perseid meteor shower, one of the brighter light shows of the year, occurs annually between July 17 and August 24. The shower tends to peak around August 9-13.

However, this article sources its information from the website Physics Astronomy, a blog-type website which posts various articles on astronomical topics. Since Physics Astronomy doesn't seem to be a particularly reputable source, I will take this information with a pinch of salt.
So, is this article true? Will you be able to see the Perseids in the daytime with the naked eye this year?


texts: Can a single impact event trigger a runaway greenhouse?

texts:
Somewhat related to this question: Is the Earth going to evolve towards Mars' fate or Venus' fate?
Human civilization may be fragile, but life itself is extremely resilient, with extremophiles and microscopic lifeforms able to withstand such harsh conditions that make the end of all life possible, but very unlikely. Once in a while articles are published, like this, listing a large enough impact event as one possible scenario, besides supernovas and gamma-ray bursts. The threshold for large enough they adopted was one with sufficient energy to boil all oceans, requiring about 5.6 * 10^26 J of energy and a impactor with a mass around 1.7 * 10^18 kg. Only 19 asteroids fit the bill, counting Vesta and Pallas.
But water vapour is known to be a strong greenhouse gas. So, couldn't be the case, instead of boiling whole oceans, a single impact could release just enough water steam to kickstart a runaway greenhouse like the one in Venus? I imagine that would happen if a substantial fraction of the atmosphere becomes steam (guess 50%). We can estimate the mass of Earth atmosphere by multiplying atmospheric pressure by planet surface area, giving about 5.2 * 10^18 kg. We can estimate the energy necessary to release a equal amount of steam, multiplying this mass by water Enthalpy_of_vaporization, and the impactor kinetic energy required, by E = mv^2/2(see below).  I assumed a 20km/sec speed that I read somewhere to be close to average for a collision with Earth (the range goes from about  11km/sec to 70 km/sec).
This gives us E ~ 1.16 * 10^25J and m ~ 5.82 * 10^16 kg for impactor energy and mass, smaller than the ones given in the link above by one and two orders of magnitude, respectively. That bring us to Hale-Bopp size range.
I'm aware Earth withstood multiple large impacts, in Late Heavy Bombardment and before, and yet the water just condensed back and so we had no runaway greenhouse induced by released steam. But as the Sun gets older and hotter, will that always be the case? Perhaps the dust released in the impacts negates the effects of water vapour in the atmosphere, but I can imagine some scenarios where the ratio of dust to vapour is minimized, like a impact in deep ocean, for example. What are the odds of something like this happened to Venus, flipping a borderline-stable Earth-like world into the runaway greenhouse we see today? Could be the case Earth is already past the same threshold, just waiting for a suitable impact to flip the switch?
I found a paper(1) discussing a sudden change from runaway icehouse to runaway greenhouse, but nothing on impact-induced transitions.
Calculations:
Python 3.6.9 (default, Jul 17 2020, 12:50:27)
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license()" for more information.
>>> m_impactor = lambda E, v: 2*E/v**2 ## From kinetic energy formula
>>> E = lambda m_water: 2.23e6*m_water ## 2.23 MJ/kg^3 = approximate heat of vaporization for water
>>> import math
>>> m_water = lambda P, g, R: 4*math.pi*P*R**2/g ## P = atmospheric pressure (about 101 kPa); g = gravitational acceleration (9,8m/s^2); R = Earth radius (about 6350 kilometers).
>>> m_impactor(E(m_water(1.01e5, 9.8, 6.35e6)), 20000)
5.822741097812724e+16
>>> E(m_water(1.01e5, 9.8, 6.35e6))
1.1645482195625448e+25
>>> m_water(1.01e5, 9.8, 6.35e6)
5.222189325392578e+18

References.

Yang, Jun, et al. “Abrupt Climate Transition of Icy Worlds from Snowball to Moist or Runaway Greenhouse”. Nature Geoscience, vol. 10, no 8, agosto de 2017, p. 556–60. www.nature.com, doi:10.1038/ngeo2994.

Sloan, David, et al. “The Resilience of Life to Astrophysical Events”. Scientific Reports, vol. 7, no 1, julho de 2017, p. 1–5. www.nature.com, doi:10.1038/s41598-017-05796-x.


texts: What would happen to black holes during the Big Rip?

texts: If the universe expands fast enough to rip atoms apart, what will happen to black holes? Would they rip apart too? What would that be like? Would they expand along with the universe's expansion, or would their gravitational force be able to resist it?


texts: Where did the Sun get hydrogen to work with if it is in the 3rd generation of stars?

texts:
As I see here, the Sun belongs to the Population I group of stars, which is the 3rd generation of the stars in our universe. 1st generation stars are Population III, 2nd generation are Population II, and 3rd generation are Population I.

When the 1st generation (Population III) of stars died, that means most of the hydrogen was burned to helium. Stars die when there is no hydrogen left. Later, the 2nd generation of stars (Population II) appeared and they fuse another portion of hydrogen into heavier elements.

If 1st and 2nd star generations burned hydrogen to helium and more heavier elements, then shouldn't like 90% of all universe hydrogen already be converted to helium and something else? If yes, then there should not be enough hydrogen to make the Sun.

UPDATE 1

Thanks for all your answers. They are very useful. Now a new subquestion appeared.
When the star dies, like our Sun, it sends out external layers and core becomes white/other dwarf. In this case, new star can be formed only from the hydrogen from the external layer. The questions what is the percentage of initial star hydrogen after burning it to helium goes from this external layer to outer space?


texts: Is it possible that some of the confirmed exoplanets are in fact primordial black holes?

texts:
Recently astronomers discovered a black hole lying just 1000 light-years from Earth, in a triple star system (HR 6819) that can be seen with the naked eye. This discovery has increased the possibility that there are many more black holes hiding in nearby star systems.

However, we have not detected any primordial black hole with only planetary mass yet. What if some of the discovered exoplanets are actually the hypothetical primordial black holes?

A exoplanet detected through the transit method are very unlikely to be black holes due to its extremely small size not possible to produce detectable dimming. But how about the exoplanets detected through radial-velocity method? A primordial black hole with 5 Jupiter mass would have the same effect on the motion of the host star as a exoplanet with 5 Jupiter mass.


texts: Where can I find permissibly licensed database of stars and constellations?

texts:
I'm working on a game engine of sorts, and one thing I did for a previous game which was under the GPL was I had a night sky with the constellations visibly connected and I'd like to do that again.  I don't know where my data set originally came from but I do have a note that it was GPL, so it likely came from Stellarium, which is GPL licensed.  For this I need the dataset to come from the public domain, us government, permissively licensed, etc.

If possible I'm after the coordinates in the sky, brightness, maybe class and name, but also some mechanism to know which stars would be connected in classic constellations.

I've found some data sets which contain the star data I'm after, though they would take some effort to parse and pull out the data I'm after, but I can't seem to find anywhere online, except stellarium, which provides star constellation connectivity. Any ideas?

I've gotten this list so far from what I can tell:

Stellarium: Has everything. GPL (Really restrictive) But has things like a constellationship file, i.e. https://github.com/Stellarium/stellarium/blob/master/skycultures/western/constellationship.fab
HYG: Great database, has everything I want, but licensed under restrictive CC 2.5 SA https://github.com/astronexus/HYG-Database No Constellations
BSC5: No explicit license, but created from NASA/ADC, data should be public domain.  Good data set, difficult to parse, no star connectivity.  (Best candidate for raw star database)
UCAC5: Navy (public domain), data set not geared for visible light.
HIP (used by Stellarium for stars) - Difficult to parse through, ESA (correct license) http://cdsarc.u-strasbg.fr/ftp/cats/I/239/hip_main.dat.gz

Are there any standards of what stars make up the primary connections in constellations?  Or does it just happen to be whoever puts it together?

P.S. If I ever do succeed, I will release the database (probably 3-4k stars is all I will need with connectivity) under the MIT license to permit inclusion in any commercial or noncommercial projects.


texts:
I'm surprised that this question hasn't been asked before (here or on Physics), to the best of my knowledge. It's one that I might have asked when I was a bit younger, and one that I think other people will ask.

Anyway, it's clear that Saturn's rings won't form a moon, and the same is likely to be true for other ring systems. However, I'm guessing that they won't last forever (it's just a guess).

How long do planetary rings in general last? What mechanisms could cause them to dissipate/fall apart/end? I'm guessing the Poynting-Robertson effect could come into play, but I'm not sure.

And for anyone curious, yes, I checked just for the fun of it, and Yahoo Answers had a bunch of really, really bad, unsourced and most likely inaccurate answers (given that there was no consensus), ranging from '3 million years' to '13-18 billion years' to 'forever'.


texts: How can I estimate how long will a main sequence star stay on the main sequence given its temperature?

texts: I have a generic zero-age main sequence star. The only thing I know about it is its temperature. How can I estimate time time it spends on the main sequence (in millions of years)? I've seen equations that estimate it based on mass, like $L=M^{-2.5}\cdot10^{10}$, but I do not have the mass, only the temperature. Is there any definite formula (accurate to a reasonable degree) for this?


texts:
As part of my physics project, I investigated the relationship between a light bulb's illuminance and the distance from the measuring device to it.

Illuminance was measured in flux, and distance in metres.

I derived an inverse relationship. First of all, can I check if this is the relationship I was supposed to get? (I understand that there is an inverse square relationship between luminosity and distance, but since illuminance is measured in lumens/sq metre, then I think an inverse relationship is correct...)

Next, using this relationship, I need to extend my project so that I can find distances to stars using this relationship.

As long as 2 stars have a similar mass/size/density, then I can use this relationship to find the distance to the 2nd star as long as the illuminance of the stars are known, and the distance to the 1st star is known.

So I tried to do this with an example - the Sun and Tau Ceti, as they are reasonably similar.

I have values for the Sun's distance/illuminance - $1.5\!\times\!10^8 $km, $1.2\!\times\!10^5$lux.
I now am trying to calculate the distance to Tau Ceti using this method.... But nowhere can I find a value for its illuminance.

Please can someone help me with this? Is there a way to convert from apparent magnitude or luminosity to illuminance?

Thank you

EDIT: The method I want to use from the inverse relationship is:

$$d_2=d_1l_1/l_2$$

where $d_1$ and $l_1$ are the known distance and illuminance of the 1st star, whilst $d_2$ and $l_2$ is that of the second, and $d_2$ is unknown.


texts: Why does the Gaia color-magnitude diagram have this shape?

texts:
I was looking at the visualization tools of Gaia, and tried plotting a color-magnitude diagram. On the right hand side of the image, photGMeanMag (mag) (the amount of light) is plotted as a function of BpRp (mag) (the color).

I was surprised by the distribution of the points in the plot, since I was expecting to see something like the main sequence of the HR diagram, like in this plot:

Why are the points distributed in a sort of wedge, rather than along a diagonal line?


texts: GW190521 black hole merger total mass calculation and missing mass, how does this happen?

texts:
I have just read an article about that black hole merge event (it's in Italian):

Sette miliardi di anni fa, due mostri si unirono

What made me curious is that the article tell that a 66 solar mass black hole merge with a 85 solar mass black hole to form a 142 solar mass black hole.
If the datas are correct, that suggests to me that during a merge event, matter inside a black hole is ejected (66+85=151).
Is that (mass ejection) possible? I have always thought that escape form the inside of a black hole is impossible.
I suppose it is somewhat possible if we think about Hawking radiation, but due my amateur knowledge level I have not been able to dig into this very much.


texts: What is the relation between between hardness ratio(HR) and spectral state of a X-ray binary star system?

texts:
When reading about hardness ratio, it shows that depending on the hardness ratio spectral states of a binary star can be divided into soft, hard states. So how the HR value changes with respect to the spectral state?
And when telling about hard state and soft state, what do we mean by this statement.


texts: Is our universe noisy or silent? Can people hear them? When astronauts are out of the Earth and near an explosion, can they hear its noise.


texts:
Gliese 581 c is described as one of the most promising exoplanets to sustain life. In this New York Times article, Dimitar Sasselov says: "It’s 20 light-years. We can go there."
https://www.nytimes.com/2007/04/25/science/space/25planet.html
What does he mean by that?
It would take roughly 40000 years with the fastest thing we've got today if I'm not mistaken?
I do understand that he's not talking about a crewed space mission!


texts: Is there a radius inside of which objects are (doppler) blue shifted?

texts:
The Andromeda galaxy is about 600,000 light years away and is doppler blue-shifted.  Are there objects closer to us than Andromeda which are red-shifted?  Are there objects farther than Andromeda that are blue-shifted?

I'm curious as to whether there is a more or less uniform radius from earth outside of which objects are generally all moving away from us, and inside of which objects are generally all moving towards us.


texts: Help with Converting Right Ascension and Declination into 3D Vectors

texts:
Using the Right Ascension and Declination values from the HYG Database from The Astronomy Nexus:

Right ascension is measured in hours and declination is measured in degrees.

1 - Convert right ascension and declination into degrees

float getAngle(float value, char units)
{
    if (units == 'h')
    {
        // 15 degrees = 1 hour
        // 360/24 = 15

        return value * 15;
    }
    else
    {
        return value;
    }
}

When right ascension is inputted, the input will get multiplied by 15, because RA is measured in hours.

When declination is entered, the input value is returned, this is because Dec is already measured in degrees.

2 - Work out the vectors

Two isosceles triangles can be derived, one for the horizontal plane and another for the vertical plane. The angles are alpha and delta, right ascension and declination respectively. Using these angles and knowing that some of the distances are equal to the distance between the centre of the celestial sphere to the star, the following code should work.

Vector3 getVectors(float RA, float Dec)
{
    float x = distanceStars;
    float z = (2 * distanceStars * (Mathf.Sin((getAngle(RA, 'h'))/ 2)));
    float y = (2 * distanceStars * (Mathf.Sin((getAngle(Dec, 'd'))/ 2)));

    return new Vector3(x, y, z);
}

Unfortunately, my code doesn't seem to work as this is the output that I am getting:

As you can see, the stars (white dots) are not spreading out evenly around the Earth (which is the black sphere in the middle).

It would be great if someone could help me with this.


texts: What is the closest one could get to a black hole and still stand a realistic chance of escape?

texts:
This question is inspired by the one asked here:

Is there any stable orbit around a black hole?

and I note that (though I've heard of these before) there are, apparently, 3 important fundamental "zones of inescapability" with regard to a black hole that can be identified on the basis of physical principles and which can be defined by the Schwarzschild $r$ coordinate and its critical radius $r_S$: one of these is, of course, the event horizon, at $1\ r_S$, from which no escape at all is possible. The second one is the "photon sphere", here referred to as the zone at the "innermost bound circular orbit", at $1.5\ r_S$, at which a photon (particle of light) traveling at right angles (i.e. constant $r$-coordinate) can complete one circuit around the hole and within which only impossibly powerful rockets could get you out. Orbits above this point but below the "innermost stable circular orbit", out to $3\ r_S$, are possible but require a continuous application of (likely ungodly large) engine propulsion. Above this, more-or-less normal orbits are possible, though they precess more and more rapidly as the general-relativistic effect that is incipient with the orbits of the planets becomes fully mature in these environs.

Now, the trick, of course, is the qualifiers regarding propulsion: "impossibly powerful", "ungodly large" which basically are likely words for "it can't be done with realistic technology". Which, then, of course, is what begs this question - what can be done with such?

For my "realistic" scenario, a generous interpretation will be used which is "a matter-antimatter fueled spacecraft with a wet mass 100x its dry mass". The goal is for the craft to execute one of two manoeuvres:

lower itself into an orbit around the hole, orbit a few times, and then take off again (so it must have fuel for both trips) and return to where it launched from, and
establish a "slingshot" trajectory around the hole that takes it as close as possible before shooting it back out into space (this may be easier?).

I consider this "reasonable" because any travel to a black hole will already be an extreme problem in interstellar travel, much less the kinds of targets that will already be in use for this kind of proposal which will almost surely have to be supermassive black holes to be interesting and thus requiring travel at least to the center of the Galaxy (~200 000 Pm) if not intergalactically (distances of many Zm), with tera- or petaseconds of travel time (thousands to millions of years) at least, or something like wormholes if they are possible. Hence I presume the technology to make such a craft will be readily available to any civilization that is capable of pulling this feat off to begin with.

The reason we need supermassive black holes is tidal forces and ideally we want to get as close to the horizon as possible. Possible targets may be Sagittarius A* and M87* (the black hole that the picture was taken of), both supermassive black holes, the latter much more so still, at the centers of galaxies. However, M87* presents the additional challenge of an extremely active matter environment around it, which may foreclose that possibility even further, though even Sgr A* might be too much. Hence, failing that, we can just consider a hypothetical black hole of mass equal to either, that is suitably "quiet", if nothing else is possible. (or perhaps imagine that with M87*, it has entered a quiescent period by the time the ships arrive as compared to the time when it sent the images we saw)

What would be the closest approach possible, in both scenarios, while still having enough fuel to get away (or not being destroyed by other effects)? We should also assume that the craft starts out at great distance where gravity is hopefully unremarkable, but do not budget for more than 250 Ms shipboard total mission time (a bit less than 2893 days of 86.4 ks each), measured from time of release from mission departure station to time of return.


texts:
A black dwarf is a hypothetical object that is the end result of the cooling of a white dwarf. None yet exist, because there hasn't been enough time in the age of the universe for them to cool down sufficiently. The oldest known white dwarfs are at ~3800 K and would glow orange. It will take many billions of years before any are cool enough (~750 K I believe) to not emit light visible to the naked eye.

So, what would a black dwarf look like? I'm assuming it is illuminated by some other source of light such as a star.

The question could be rephrased: What is the visible absorption (and emission) spectrum of electron degenerate matter?

This is my attempt at an answer: Probably not black. A lot would depend on whether electron-degenerate matter behaves as metallic or dielectric, based on its band-structure. My guess would be metallic, considering the electrons are dissociated from their parent nuclei in electron-degenerate matter.

Because the surface gravity of a black dwarf would be so strong, I'd expect the surface to be very smooth and relatively free of Lambertian scattering.

From these two features, I'd make a tentative prediction that a black dwarf would appear as a mirror-like sphere.


texts: Were all first generation stars of same mass and size?

texts: If the density of the matter from which the first generation stars were made was homogeneous, so during the star formation nuclear fusion would have started around when they all reached to same critical mass and core temperature, once fusion started the outward pressure would have counteracted on inflow of matter so all stars should have been of similar size


texts: Light pollution and apparent magnitude of objects in solar system

texts: Dust along the ecliptic scatters light, causing “light pollution” throughout the orbital plane of the solar system. If an object like, say, the sun's corona, had its brightness measured from above the ecliptic, would the measurement reflect that?


texts: Astronomy as a profession for moderately good software engineer

texts:
I'm a software engineer (a moderately good one). I like programming, mathematics and I love everything about astronomy.

What would be best way to go ahead?


texts: A simple question: is there light on the surface of Venus?


texts: How to fit lorentzian to lyman alpha cross section?

texts:
This is my data and purpose of this assignment is to fit a lorentzian to this data.
And.. i dont know how to do that....:(

First, there is sharply lowered region.
Couple of hours of googling isn't helpful..
Second, should I fit lorentzian for 2 times?
There are two peaks.


texts: How well would the Moon protect the Earth from an Asteroid?

texts:
Would the Earth fare better if the Moon blocked the meteor, comet, rogue planet, or otherwise rather than a direct impact? At what point would the Moon's debris would be an extinction event?

The limit of the impactor is the size, angle of attack, or composition in which to moon would no longer be of any protection.


texts: Accelerations on the Moon due to its orbit and its spin

texts:
A gyroscope is placed on the equator of the Moon. It detects among others 2 accelerations: one due to the orbit and one due to the spin. What is the module of the second one compared to the first one?
(the effects due to the tilting of the orbit, the librations, the Sun... are neglected.)


texts: Meteor size and frequency distribution for Earth vs. moon or other planets?

texts:
I struggle to find an answer to the at-first-sight simple question if or how the size distribution and frequency distribution of incoming rocks hitting the ground or the atmosphere vary from Earth, to the Moon, to other planets (e.g. Venus, Mars, Jupiter). Let's limit ourselves to observable events:

For Earth, I would like to consider all observable objects hitting the Earth atmosphere, see Baggaley: The size distribution of large meteor bodies (1978)
For our moon, we would only consider objects which leave a craters e.g. observable by Earth-based telescopes, see Werner et. al.: The Near-Earth Asteroid Size–Frequency Distribution: A Snapshot of the Lunar Impactor Size–Frequency Distribution (2001).
For gas planets, I would assume that there are more observations like Something Just Smacked Jupiter

I am after some statements for meteors for our Moon, and other planets similiar to the comprehensive answer on How often do meteorites hit the Earth?


texts:
Can someone perhaps assist me in identifying the yellow object between δ Ori and η Ori? I took an image of this object, but I can't identify it.

It can be seen on the image below, hosted on the Eagle Creek Obs. site. (http://www.eaglecreekobservatory.org/eco/doubles/images/ori.gif)

It's the orange object on the constellation line between 28 and 34.


texts: Is there an API to obtain Solar System body axial tilt data?

texts:
I would like to get information about rotation period and axial tilt for main Solar System bodies and major moons, over time.
Is there an API availabe for this?
I've checked JPL's Horizons API and CSPICE toolkit, but as far as I've seen axial tilt and rotation period are constants, not values that can be retrieved for different points in time.


texts: How can gravity lead to the Big Crunch scenario?

texts:
According to modern cosmology, space is expanding, causing proper distances (but not comoving distances) to increase between galaxies. In the Big Crunch hypothesis, gravity halts and reverses the expansion of the Universe, causing all matter to collide and eventually form a single black hole. This gives way to other oscillating universe hypotheses, which generally propose that the conditions in a compressed Universe would be the same as those during the Big Bang, leading a cycle of expanding and contracting universes.

Ignoring the problems with entropy in reverting the Universe to Big Bang conditions, how can gravity be the cause of a Big Crunch in the first place? Specifically, gravity (to my knowledge) only curves space; the idea that it can revert the Universe to the conditions of the Big Bang seems to imply that gravity actually can contract space. Is this actually the case?

If not, the gravitating objects should be moving through a comoving coordinate system, so space itself would not actually be contracting. As far as I can tell, we would have all the Universe's matter compressed within a single point in space, instead of space itself contracting. This should be completely unlike the Big Bang, when space was far less dilated than it is now. If this is in fact what the Big Crunch hypothesis describes, than I'm utterly confused as to how an oscillating universe could work in such a situation.

Am I mistaken, or do the Big Crunch and oscillating universe hypotheses imply that gravity actually contracts space (as in, the comoving distances of gravitationally attracting objects would not change)? If not, how could gravity possibly lead to these scenarios?


texts: Is there any orbit at which the Roche limit can be "felt"?

texts: Do any of the planets have a Roche limit that is strong enough to be felt by an astronaut whilst in orbit?


texts: How did they estimate the mass of Sgr A*'s companion G2 without knowing what it was?

texts:
Gizmodo.com's Astronomers Spot Unprecedented Flashes From Our Galaxy's Black Hole mentions Sgr A*'s companion gas cloud G2.

That Wikipedia article says:

  First noticed as something unusual in images of the center of the Milky Way in 2002, the gas cloud G2, which has a mass about three times that of Earth, was confirmed to be likely on a course taking it into the accretion zone of Sgr A* in a paper published in Nature in 2012.

but later in the same section says:

  Nothing was observed during and after the closest approach of the cloud to the black hole, which was described as a lack of "fireworks" and a "flop".[57] Astronomers from the UCLA Galactic Center Group published observations obtained on March 19 and 20, 2014, concluding that G2 was still intact (in contrast to predictions for a simple gas cloud hypothesis) and that the cloud was likely to have a central star.

and

  Professor Andrea Ghez et al. suggested in 2014 that G2 is not a gas cloud but rather a pair of binary stars that had been orbiting the black hole in tandem and merged into an extremely large star.

Question: Without knowing what this object is, how could its mass be determined to be only 3 times Earth's mass, way too small to measurably perturb the other stars in close orbits around Sgr A*?

Relevant and interesting reading: The Story of a Boring Encounter with a Black Hole


texts: Is every star formed in an open star cluster?

texts:
As far as I understand, an open cluster is formed from a single molecular cloud, with each star in the cluster having roughly similar age and properties.

Our Sun is not part of any star cluster, but it could have been formed in one. Is every star formed in an open star cluster, or can they develop independently?


texts: How do proto-planetary nebulae gain momentum?

texts: currently I am curious about proto-planetary nebulae. At first I was curious about how planets spin but I read in a couple of articles that they got their spin from when they were in a proto-planetary nebula and the momentum stayed because of conservation of momentum. But now a new question arises, where exactly do proto-planetary nebula's get their momentum from? I've searched this up on the internet but I haven't really gotten a clear response.


texts: Is there some upper limit in the moon size distribution?

texts:
The planets range hugely in sizes, Jupiter is more than 300 times as massive as Earth. But the sizes of the large moons seem to be uncorrelated with the sizes of the planets they orbit. Why doesn't Jupiter have a giant moon with a mass hundreds times larger than the Earth's Moon's?

Among gas giants, Saturn at a third of Jupiter's mass has the largest of all moons and Triton way out at 30 AU Neptune is the 7th largest of all moons. The theories of the origin of the moons seem to vary much too, without explaining any upper size limit. The Moon was formed by an impact. Triton and maybe Titan were captured. Other large moons were formed along with their planet. Is this upper size limit on moons imaginary, coincidental or is there some reason for it to be this way? I of course mean a hypothetical expected upper size limit for exomoons in general, not only statistics about the Solar System moons.

The wonderful illustration here is from Emily Lakdawalla at The Planetary Society, it is now my desktop background.


texts: What reason is there to doubt the existence of the hypothesised planet Tyche in the far distant solar system?

texts:
Please note, I do not necessarily believe in its existence, just after a scientific (hence, non-Wikipedia) reasoning to doubt the planet's existence, other than the "we would have seen it" argument.  I am, nor are the articles below, referring to the alleged 'Nemesis' planet, as in the question What are the current observational constraints on the existence of Nemesis?

According to the article Astronomers Doubt Giant Planet 'Tyche' Exists in Our Solar System (Wolchover, 2011), the planet Tyche is a hypothesised massive planet

  by John Matese and Daniel Whitmire of the University of Lousiana-Lafayette, is not new: They have been making a case for Tyche since 1999, suggesting that the giant planet's presence in a far-flung region of the solar system called the Oort cloud would explain the unusual orbital paths of some comets that originate there.

Further information is found in the scientists' research paper Persistent Evidence of a Jovian Mass Solar Companion in the Oort Cloud (Matese and Whitmire, 2010), describe the hypothesised planet to be

  of mass  1 − 4 M(Jupiter) orbiting in the
  innermost region of the outer Oort cloud. Our most restrictive prediction is that the orientation
  angles of the orbit normal in galactic coordinates are centered on
  , the galactic longitude of
  the ascending node = 319 and i, the galactic inclination = 103 (or the opposite direction)
  with an uncertainty in the normal direction subtending  2% of the sky.

They posit that the elongated orbit of Sedna is a result of the presence of this planet.

So, the question, what reason is there to doubt the existence of Tyche?


texts: Relationship between the Casimir force and dark energy

texts: This is related to another question I asked. Might we actually be observing dark energy when we do experiments that verify the Casimir effect?


texts: What is the most oblate astronomical object known?

texts:
There are some very vast spinning objects out there.   How high can the eccentricity get, or, which I think is easier to understand, how small can the ratio between the polar and equatorial diameter be?

For clarification, I'm asking about objects composed composed of particles (not some boundary or field etc) and that are devoid of non-microscopic empty spaces. I.e. a planet or star etc; not a galaxy or rings or a heliosphere etc.
For the purpose of this question, I just don't find galaxies or nebulae interesting because they can have almost any shape.

I'm talking about structures that are oblate spheroids essentially.


texts: How to conceptualize the strong equivalence principle (SEP) in the context of stellar motion in an extragalactic galaxy?

texts:
In the theory of general relativity, the SEP states all the laws of nature are the same in a uniform static gravitational field and the equivalent accelerated reference frame. I've been reading the press release of a recently published paper (the paper itself discusses Modified Newtonian Dynamics but for the purpose of this question, MOND does not need to be addressed), and I'm confused by a statement that is made, so I guess I don't understand the SEP after all. Here is the statement and the article page:
"One of the core tenets of general relativity is the strong equivalence principle, which states that the motion of stars in a galaxy should be independent of an external uniform gravitational field.”
Press release: https://www.forbes.com/sites/drdonlincoln/2020/12/20/study-questions-both-dark-matter-and-einsteins-theory-of-relativity/?sh=2b487d402b7a
I don't understand how the SEP negates the concern of a gravitational field of other bodies but I also don't know what is meant by "external" here so I guess we can start there.
Maybe I am too used to thinking about gravity much more simplistically because I am more used to the Newtonian context but wouldn't all bodies that have a mass need to be considered to properly determine the gravitational "force" on a body? For instance, you'd need to know the contribution from every mass in the universe to know the value of the gravitational field exactly, but this level of precision is not necessary. Although it is not necessary, wouldn't it then be imprecise to state the "motion of stars in a galaxy should be independent of an external gravitational field"? How can a gravitational field be external anyway? From what I understand the gravity is considered local in the sense that there is a gravitational field at my precise location; however, this field can be influenced by objects very far away. I really don't understand how the motion of stars or gas in the outskirts of galaxies can not be influenced by the mass of external bodies. I guess I have myself very confused. I keep trying to sort this out myself, but here we are. Thanks in advance. Conceptual explanations are much more helpful to me than math. I add the math later after I understand the basics of what's going on.


texts: I understand what differentiates elliptical galaxies from spiral, lenticular, and irregular galaxies. What is the difference between a compact elliptical (cE) galaxy and a dwarf elliptical (dE) galaxy? Are there criteria which separate these two subtypes?


texts: When an object revolves around an oblate planet, in near-circular orbit, does the oblateness ALWAYS make it moves FASTER?

texts:
tl;dr: The orbital period for a Keplerian orbit of a small body around a massive one with standard gravitational parameter $\mu=GM$ and semi-major axis $a$ is given by
$$T = 2\pi\sqrt{\frac{a^3}{\mu}}.$$
I'm asking if for near-circular orbits the rise times of the object will always come earlier than predicted by a Keplerian model when the central body is oblate, i.e. has a positive $J_2$.

Assuming that we want to make a "long-term" prediction of the rise-time wrt any given observation point on a planet, of an object circling said planet, assuming that we want to make a rough estimate using the Kepler model. Is it correct to state that the said rough estimate is ALWAYS lagging because we ignore the oblateness of the planet? By "lagging", I mean we should observe the rise somewhat BEFORE the predicted calculation, but always before?
A side question: Assuming we know the J2 zonal coefficient and the orbital elements at epoch (t0), is it correct to say that the prediction "error" at time t is simply linear with delta_t (t=t0+delta_t)? (the point is that if your answer is yes, then I can use the simplified model w/o making any error compared to a J2-aware model).
Note: For now, I am not interested in a real-life situation, where the orbit is perturbed by other forces than J2.


texts: Why must all magnitude systems have a reference point?

texts:
Let $f_*$ and $f_0$ be the observed flux of a star and a reference flux in a particular spectral band, and let $m_*$ and $m_0$ be their respective apparent magnitudes. Then the star's magnitude is given by $m_*-m_0=-2.5\log(f_*/f_0)$.
Encyclopædia Britannica maintains that

All magnitude systems must have a reference, or zero, point.

Why is this? It seems to me that one could just say $m_*=-2.5\log f_*$ and move on, no?


texts: Hypothetical: Is the interstellar wormhole too close to the black hole?

texts:
Most reasonings about hypothetical wormholes say, that you need an extreme amount of energy to keep them open.

Given that, isn’t the worm hole too close to the black hole gargantua? The gravity between the two would make the spin and eventually collapse together?


texts: If jupiter was the densest planet on the solar system with low volume, could it be a star without multiplying its mass 80 times?

texts: Astronomers or Scientists say that Jupiter's Mass would have to be multiplied 80 times to become a star (M Mass). What if Jupiter was denser, would there be thermonuclear energy because of the density?


texts: Is our supercluster part of a galaxy filament?

texts:
It was discovered recently that the Milky Way galaxy lies inside a supercluster named Laniakea Supercluster. On the other hand, wikipedia says that galaxy filaments are "the largest known structures in the universe"
https://en.wikipedia.org/wiki/Galaxy_filament

My question is: is the Laniakea Supercluster part of a galaxy filament, and if so, what's the name of the galaxy filament?


texts: Attempting to determine astronomy values in a parameter file

texts:
Recently I inherited quite a few old QuickBasic programs which perform various astronomical calculations.  I'm attempting to understand these programs and rewrite some of them in Python.  I do not have a deep background in astronomy.

A number of the programs take a parameter file as input, YEAR.DAT.  Below are 5 years of these files (each column represents one file).  I need help in figuring out the various data values.

YEAR.DAT

year        2001         2008         2009         2010         2011
delta t      66           65           66           66           67
tilt         23.43909     23.43818     23.43805     23.43799     23.43786
dow           1            2            4            5            6
gst           6.71430      6.66860      6.71839      6.702509     6.68659
x1          105.690      330.340      310.959      291.631      272.303
bs           84           90           88           87           86
fs          301          300          298          304          303
x2          357.765      356.959      357.689      357.433      357.177
x3          354.289      193.159      335.720      105.105      234.489
jd      2451910.5    2454466.5    2454832.5    2455197.5    2455562.5

I believe that all the values which are time dependent are for 0:00 hours on Jan. 1 of the year given.

Here are the values I think I've figured out:

tilt is the obilquity of the ecliptic
dow is the day of the week, where Monday is day 1
bs is the number of the day of the year when British Summer Time (BST) begins
fs is the number of the day of the year when BST ends
jd is the Julian day number (of 0:00 hours Jan. 1)

Values I'm unsure about:

delta t is some sort of time delta, but I don't know what
gst seems to be Greenwich Mean Sidereal Time, but for what moment?
x1, x2, and x3 I'm clueless about

Here are my questions:

What might delta t be?
Is gst in fact Greenwich Mean Sidereal Time?  For what moment?
What are x1, x2, and x3?  (This is low-priority question.)
How can delta t, gst, and, perhaps other values, be determined for
    2018, 2019, ...?

Any help will be greatly appreciated.

Roger House


texts: Can Milky Way be seen in South hemisphere only?

texts: I heard something(long time ago) that Milky Way can be seen in South hemisphere only, is it true? If so, what is the reason?


texts: How does the Astronomical Twilight depend on the night length or the day length?

texts:
Some references say that the Astronomical Twilight starts at the last 1/8th of the night length, so it is long in Winter and short in Summer;

But by using the 18 angle (sun elevation below the horizon) the twilight is long in Summer and short in Winter!

What would be the correct method proven by observations for all days and all locations?


texts: Spectrograph Arrangement, Noise, and Fourier Domain

texts: If the transform of a line profile extends out to the Nyquist frequency such that we cannot see the noise level, what could we change in our spectrograph arrangement that would allow us to see the noise level in the Fourier domain?


texts: How can we tell the difference between matter and antimatter by observation in space?

texts:
I just was wondering and searching on the internet with little luck in the topic. On Antimatter Wiki they tell the observable universe is built up by matter. I read antimatter can be detected in cosmic rays in very small quantity. But let's go in theories and our practical knowledge, my question is:

How could we detect if a set of objects (like an entire galaxy) is completely built up by antimatter? Since matter and antimatter look like "mirror image" on current, could they form a similar object which is indistinguishable from "our" matter objects? I mean, would spectrum analysis for example show that it is not matter, but built up by antimatter?

Linked question: Are we sure about that what we see in universe is matter?


texts: If we pumped an large EMP into High Earth orbit and it detinated would it act as a shield against solar radiation?

texts:
Could this pulse potentially shield spacecraft from harmful solar radiation.

I was assuming an EMP would have little effect as a radiation shield because the pulse would not be rapid enough to counter act the suns energy

f we pumped an EMP into high earth orbit and it detinated would it create a pocket of radiation-free space


texts: Was there ever a meteorite with a measurable gravity?

texts: Did we ever find a meteorite (a meteoroid that has fallen down to Earth) whose mass, density and gravity were high enough to demonstrate their gravity similar to the Cavendish experiment, so that something was attracted to it?


texts:
I feel like this is going to be a duplicate of this question, Astrophotography - Unable to achieve focus with current setup.

However, the suggestions to go to a straight adapter doesn't apply in my case. I have a Canon Rebel XT, with a 2" 48mm adapter https://www.amazon.com/gp/product/B00T3P5ABC/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1, on my Orion XT10 dob, http://www.telescope.com/Telescopes/Dobsonian-Telescopes/IntelliScope-Dobsonians/Orion-SkyQuest-XT10i-IntelliScope-Dobsonian-Telescope/pc/1/c/12/sc/27/p/102013.uts.

When I attach the adapter to the camera and focus on a planet or star, I can't achieve focus. My focus adjuster on the scope can't get close enough. It's as if the camera is just out of range of the focuser.

I'm not sure what the issue is and would love to hear some suggestions. I'm just a noob getting started and have a lot to learn so I apologize at all the beginner questions here.


texts:
When a satellite takes measurements of the radiation from different patches of the sky to construct a map of the thermal radiation of the universe on the surface of a sphere, it would also contain radiation from the Milky Way, for one.
A big patch of the map will have noise due to the objects in the Milky Way or from other sources.

How does one know where and how much noise to remove while constructing the map?


texts: Why are stars more metallic closer as you move closer to the galactic bulge?

texts: As I see it, most of the stars in the galactic bulge are Population I stars. However, as one moves farther from the galactic bulge, star metallicity decreases. In fact, halo stars are almost entirely Population II stars. Why is this?


texts: What is in the brightest area of the night sky?

texts:
When I look up in the night sky, I assume the brightest area, commonly known as "The Milky Way", is the Galactic Center of our home galaxy.

But then I stumbled upon this illustration, which seems to suggest what I'm seeing is (and by far) not the Galactic Center. In fact, from this picture it looks like the brightest area might be the closest spiral arm (on the opposite direction of the galactic center).

I know this illustration is not meant to be precise, so it's probably not the closest spiral arm either.

So, what am I really looking at when I stare at the brightest spot of the night sky? What percentage of it would be stars, planets, and nebulae? And what's the farthest object I can see in that direction with my unaided eyes?


texts: Relation between age and spectral type of star

texts:
Does the spectral type of a star change as it ages? Since its temperature changes, I suppose the spectral type should also change, but I'm a bit confused about this.
Also, is the spectral type entirely based on the star's spectrum? If yes, then why do we say O types are hotter and M types are cooler. I mean the statement is true but why not say that O types have very few lines and M types have a large number of lines? Or is it just a phrase stuck through history just like O type being early and M type being late stars (although this one is wrong)?


texts:
In many HR diagrams, the asymptotic giant branch includes a wiggle. I thought that these are thermal pulses, but now I realise there are many thermal pulses, and I find only one wiggle. The wiggle could be dredge-ups, but there could be more dredge-ups, as there are way more than one thermal pulses.
What is the wiggle in the AGB phase?
Thermal Pulses:

Wiggle on a solar mass star's AGB path:

Wiggle on a 5 solar mass star's AGB path:

All images from AGB Wikipedia article.


texts: How often are radio telescopes used to measure parallax? When was this first done?

texts:
The abstract of A magnetar parallax (also in MNRAS) says:

TE J1810-197 (J1810) was the first magnetar identified to emit radio pulses, and has been extensively studied during a radio-bright phase in 2003−2008. It is estimated to be relatively nearby compared to other Galactic magnetars, and provides a useful prototype for the physics of high magnetic fields, magnetar velocities, and the plausible connection to extragalactic fast radio bursts. Upon the re-brightening of the magnetar at radio wavelengths in late 2018, we resumed an astrometric campaign on J1810 with the Very Long Baseline Array, and sampled 14 new positions of J1810 over 1.3 years. The phase calibration for the new observations was performed with two phase calibrators that are quasi-colinear on the sky with J1810, enabling substantial improvement of the resultant astrometric precision. Combining our new observations with two archival observations from 2006, we have refined the proper motion and reference position of the magnetar and have measured its annual geometric parallax, the first such measurement for a magnetar. The parallax of 0.40±0.05 mas corresponds to a most probable distance 2.5+0.4−0.3 kpc for J1810. Our new astrometric results confirm an unremarkable transverse peculiar velocity of ≈200km s−1 for J1810, which is only at the average level among the pulsar population. The magnetar proper motion vector points back to the central region of a supernova remnant (SNR) at a compatible distance at ≈70 kyr ago, but a direct association is disfavored by the estimated SNR age of ~3 kyr.

This reports the first radio astrometric determination of parallax "for a magnetar".
Question(s):

How often are radio telescopes used to measure parallax? Is it done semi-regularly, or is this a rarity, used only in special cases?
When was the first time that the parallax of a distant object (e.g. outside of our solar system) measured for the first time using radio astrometric techniques?

Unfortunately "first time" is ambiguous, it could mean first time ever for this kind of measurement, or first time for a given object i.e. nobody knew the parallax of that particular object until the radio astrometric value was determined. It may be too much work to try to answer both. If that's the case, just indicate which kind of "first" is being reported.


texts: Would DCBH form supermassive stars first before BH formation

texts:
In reading some literature it seems ambiguous (intentionally?) as to the path that Direct Collapse Black Holes (DCBH) take. Some papers do talk about pristine Halos collapsing to form supermassive stars (SMS) ($10^{3}$) that then undergo direct collapse. However most that I've seen do not.
So my question is, do all DCBH models assume a short lived SMS phase or can there be a runaway collapse/accretion in the core of a H cloud that collapses into a intermediate mass black hole or larger? Are there multiple routes for DCBH? This is all assuming $z>15$ and $\Lambda$CDM.


texts: Looking for a 3D animation showing the motion of the sun relative to other stars and the Milky Way, any one seen anything like that?

texts: I am looking for an animation that not only shows the inclination of the solar system respective to the plane of the galaxy, but also shows the motion of the sun relative to and in conjunction with (i.e keeping the local frame of reference of rest intact) the local group of stars. If anyone has seen such a animation or if anyone has the skill set to make this animation then it would be a great help!


texts: How can 'HD 140283' be older than the universe?

texts:
Scientists have known about the star HD 140283, informally nicknamed the Methuselah star, for more than 100 years, since it cruises across the sky at a relatively rapid clip. The star moves at about 800,000 mph (1.3 million km/h) and travels the width of the full moon in the sky every 1,500 years or so, researchers said. Its apparent magnitude is 7.223.

Previous research had estimated that the Milky Way galaxy's so-called
"Methuselah star" is up to 16 billion years old. That's a problem,
since most researchers agree that the Big Bang that created the
universe occurred about 13.8 billion years ago.

Later estimates show that the star could be as old as 14.5 billion years (± 0.8 billion years), which is still it older than the universe's calculated age of about 13.8 billion years. This is an obvious dilemma.

How can a star be older than the universe itself?


texts: First observation that the Sun and Jupiter (and friends) move around a common barycenter?

texts:
Answers to the question How did Kepler determine the orbital period of Mars? describing careful observations centuries ago got me thinking.

What was the first analysis of observations that directly demonstrated that the Sun and the outer giant planets were moving around a common barycenter rather than all planets rotating around a fixed Sun?

I am not sure if First observation that the movement of a planet or asteroid in its orbit was affected by another planet? is a completely separate question, or if this confirmation happened all at once, but I've currently asked it separately.


texts:
Planets (at least, some of them like Earth) aren't exactly spherical - but what about stars?

Is Sun perfectly spherical, for example? What may be the reasons if it isn't? What about other stars?


texts: What would happen if we stepped on the Sun?

texts: I mean in intrinsic detail. What would happen to our bodies besides "burst into flames"?


texts: Do meteor showers start slowly each night?

texts: Does the frequency of the number of visible meteors during a daily shower start off lower at the beginning only to increase as the night goes on or does the frequency remain the same for the entire night?


texts: How to decide which part of the data is analyzed?

texts: I need to get the X-ray light curve of Classical Nova V1494 Aquilae (1999 No. 2) in Outburst. It is done long time ago, but I need to this by myself. While reading the papaer about the V1494 Aquilae, they have analyzed the data belonging three seperate time intervals, then they combined the three graph to get final light curve. The final one shows a clear pick corresponding to the outburst.


texts: periodic inclination change circumbinary planet

texts:
I am studying circumbinary planets and I made a python model to calculate certain properties of the orbits, and when I calculate the inclination I get a sine wave.

I do not see a way to predict the shape of the curve in advance, despite it being a rather simple curve. Furthermore, when I plot the eccentricity of the orbit the frequencies of the curves do not align.

Would anyone know why it oscillates at this rate and amplitude?

And a side question, would anyone know some literature on the subject of circumbinary planets?


texts: What does this "web on the surface of the Sun" image reflect? What does 789 nm show us?

texts:
What is this web on the surface of the Sun? has got me thinking.

This is probably not a normal color photo.
The cooler areas are really dark!

update: Comments point out

The NSO press release says the passband is 789 nm.
"Images have been processed to remove noise and enhance the visibility (contrast) of small-scale (magnetic) features while maintaining their shape. The movie frames have been smoothed to remove noise."

Question: What does this image at 789 nm show us? Are we looking at changes in blackbody radiation or is there a spectral feature that tracks something more specific? Does intensity in the image actually track temperature directly, or reflect it in a more subtle way?

There are two videos linked in Phil Plait's Bad Astronomy article

DKIST first light high-resolution video of solar granules
DKIST First light video of solar granulation (wide angle).

From The Universe Today's This is the Highest Resolution Image Ever Taken of the Surface of the Sun


texts: Accuracy of Mercury transit calculations

texts:
What is the error range for calculations for the time of 1st-4th contacts for a given GPS coordinate on Earth?

I observed through a telescope the 3rd and 4th contacts and compared against SkySafari and Stellarium.  I found SkySafari showed contacts about 10 seconds earlier for the same location vs Stellarium.  Stellarium was very close to my measurements for 3rd contact (1-2 seconds).  To get the calculation I zoomed the GUI and tweaked the time until it was closest to the contact.  Estimates for 4th contact (transit end) are much harder to visually time.

This got me interested to know if there is a gold standard for such calculations, and what is the range of error for it.   For forecasts n years out is the error range n times more?


texts:
How do people measure the distance between the Earth and The Moon?
What methods were used? Where can I find the official data?


texts: Picking two random points in space and drawing a line between them, what is the chance the line intersects anything?

texts: I thought of this question after pondering how most of the observable universe is observable. If we can observe it, then the space between us and that object must have been unoccupied to allow the photons to travel uninterrupted.


texts:
Can one see a black hole on earth?

If a black hole does not emit light, how can one take a picture of the black hole itself?

There's some discussion that the image composed by the Event Horizons Telescope is really just an accretion disk. To "take an image" of something, you need the light reflecting off of the surface of the object within frame. The "light" in the image is really just color-mapped ...
Can one see a black hole on earth?


texts: What is the difference between azimuth angle and hour angle?

texts: What is the difference between an hour angle and an azimuth angle? When I see illustrations of the two, the angles look very similar.


texts: Why does the Solar System have no (natural) satellites of satellites?

texts: What are the conditions for a planetary moon to have a satellite of its own? How far do the Solar System's bodies fall from the necessary threshold?


texts: I saw a light moving fast what could it be?

texts:
Today, at about 23:40 in Sungkyungwan University, South Korea, a friend and I spotted a bright light.

It was brighter than nearby stars. Since I was looking at South, and at that time, the brightest star in the Southern sky was Saturn, with an apparent magnitude of about 0.9, I'm safely assuming that its apparent magnitude was brighter than 0.9. The thing was even brighter than that (I think that it was as bright as a full moon).

Its initial altitude was about 60degrees then it fell below the horizon very quickly (in about 3~4 seconds). At first, I thought that it was a comet of some kind, but It wasn`t leaving any traces in the sky, and also it was moving in sort of a random way, going from left to right to left to right and so on.

It was a point, but it was so bright that it appeared larger than Vega(I'm not talking about absolute sizes, as stars don't have sizes. I'm just saying that, just as brighter stars seem larger, this moving thing was brighter and therefore, seemed larger.)

My friend and I saw it together, and wondered what it was.

Then, after about 10 minutes, it appeared again. But this time, it came from the place where it disappeared at its first sight. It moved up to about 10degrees (altitude). When it was going up, it was going up in a parabolic path. Then, it came down but along the way, its path changed from parabola to zig zag. Going from up to down took about 2 seconds.

What could this be? This cannot have been an optical illusion because both my friend and I witnessed it. Also, it could not have been a satellite as it was too bright(almost as bright as the moon) and moved in a non-linear path (also it moved very very fast).

What could this be?


texts: Is there any definitive proof of stars being "born" in nebula formations?

texts: Is there any absolute evidence of star formation inside of nebulae (AKA stellar nurseries) as is suggested and even stated as fact by many textbooks?


texts: Orrery is a clockwork model of the solar system. I am trying to emulate one in 2D. Now, to emulate, I need to know what goes on inside. Can someone please explain the basic principle behind the clockwork? Or direct me to a resource that will explain all the machinery inside a simple Orrery.


texts: Is a planetary system star's referred to as their sun?

texts:
It's my understanding that the Sun (uppercase) is used to refer to our sun/star, because it is The Sun. Latin name is Sol, hence the Solar System. Even the tag (lowercase sun) on this post has the caption "Questions regarding the closest star to Earth, at the centre of the Solar System."

Other planetary systems have their own stars, which I understand are called a sun, in the context of that planetary system.

I've tried my best to find a credible source that exoplanets indeed have their own suns, and that the term "sun" doesn't just refer to our Sun. I have only found this one website that backs this up: http://curious.astro.cornell.edu/physics/78-the-universe/stars-and-star-clusters/general-questions/346-what-is-the-difference-between-a-star-and-a-sun-beginner

It seems to be common knowledge in the scientific community that this is correct, but I can't find anything else online to back this up. Help?


texts: Does the Earth ever cross the Moon's path with respect to the CMB?

texts:
Title says it, really. As I understand, our total peculiar motion with respect to the CMB is 631 (+/-20) km/h, in the direction of 10.5h RA; -24 deg Dec (or rather away from 22.5h RA; +24 deg Dec, right? That's close to the direction of Lambda Pegasi).

So, if we draw a line from the Moon to Lambda Pegasi, does Earth ever cross it?

I think the answer is no, but don't have enough of a model to prove it to myself.

The inclination of the Moon's orbit is 5.14 degrees, and the Earth subtends about 1.2 degrees from the moon, but the Moon's path extends off at 24 degrees above the ecliptic. So the Earth won't cross it, right?

This is all assuming that the vector info above is the total, and already accounts for the motion of the sun around the milky way. If that's not the case, please let me know.


texts: What is the longitude of the winter solstice for the various planets?

texts: I am looking for data showing the longitude at which the various planets  in the solar system reach their winter solstice.  Where could I find this data?


texts: What can Jupiter's spectrograph tell us other than its rotational velocity?

texts:
I have data of a Jupiter's image and the spectra of each pixel in the image, and I am playing around to see what I can do with it.
I have managed to find Jupiter's rotational velocity by finding the difference between the wavelengths of H-$\alpha$ line of two pixels one at the centre and another at the edge of Jupiter's data cube.
I have also managed to identify a few spectral emission lines of Jupiter's red spot.
Is it possible to do something similar and find the orbital velocity, or the distance between Earth and Jupiter?  Or is there anything else that a spectrum of Jupiter can tell us?


texts: Considering our methods of exploration, how likely is it that there are unfound planets (not dwarf planets) in our solar system?

texts: I think it's probably unlikely that there are more planets between Mercury and Mars, but out from Jupiter, there's lots of empty space between the planets. Could there be some small planet hidden out there?


texts: the initial astrometry calibration for a telescope

texts:
Generally a telescope can memorize how to point to a target with a particular RA&Dec. There is no need to calibrate every day.
How can it happen?
How to let the telescope(non-amateur) know the position of a star after it was built? For an amateur telescope, how to realize it? There should be a zero point?

How often should the calibration be?


texts: Standard Big Bang model and space curvature

texts:
Why it is said that in standard Big Bang model space curvature grows with time when the Universe expands? My intuition is that it should be the opposite.

I saw this in a book talking about the problem of standard Big Bang model and why we need inflation theory. It says that the Universe is observed to the nearly flat now. And because the space curvature grows with time, it should be even flatter in the past, which cannot be explained. But the inflation theory solves the problem because even if space is initially far from flat, inflation will smear out the curvature and make it flat.


texts: Do Stars in Galaxies also move in elliptical orbits?

texts:
There is a gravitational force between every pair of objects, because of which planets revolve around the Sun in elliptical orbits.

My question is this: As planets move in elliptical orbits with the Sun at a focus so do the stars in galaxies also orbit, and if then what are the foci of their orbits. Since the gravity of black holes is very high are black holes at any of the focus of a star's orbit?


texts: Is there an optimum orbit for a Hipparcos/Gaia-like parallax observatory

texts:
The Gaia spacecraft is currently mapping the stars in the Milky Way by parallax. It orbits in the Earth-Sun L2 Lagrange point. The parallax observed from the spacecraft orbiting around the sun allows us to derive the distances to many stars in the milky way, as well as calculate their velocities.
Assuming no cost to placing such an observatory wherever we want around the sun, what would be the optimal orbit for optimal rate of science gains?
I apologise if this seems a big vague, let me elaborate.
If you imagine placing it in a smaller orbit, the parallax of distant stars (already very small) will be made even smaller. But information about the parallax will be returned quicker.
If you place the observatory in the orbit of Uranus, you'll have a much greater parallax and will be able to measure the distances of distant stars with much greater precision. However, it will take 80 years to complete an orbit, and the rate of distance measuring would be intolerably slow.
Intuitively, I feel like there is some obvious sweet spot that would produce measurements at an acceptable rate, though I also feel like my objective function here is ill-defined.
Would a highly eccentric orbit be preferred to a circular one? Would a particular orbit optimise the 'science gains' for one particular aspect at the expense of others (such as accuracy of nearby stars traded for accuracy of distant stars, or accuracy of stellar proper motion)? If there are multiple objectives to optimise, what would  the tradeoffs be?


texts: When did we discover that our sun does not orbit its own axis but a barycenter?

texts: It's commonly known that the center of gravity of our solar system is not the suns axis, and, that the same goes for other orbital systems like Pluto and Charon, and so on. When was it discovered that our sun has an orbit around a barycenter?


texts: Re-align HST narrowband images to GOODS-S

texts:
I want to re-align the image data of filter F658N to GOODS-South catalog with TweakReg. I used the external catalog of image data generated by SExtractor as input catalog (threshold is set to 5.0) because of the artifacts. Although I can find some matches by visual check using ds9，the TweakReg can't detect the matches between the input catalog and the reference catalog to align my images to the GOODS-S catalog.
I have tried different searchrad and threshold, but it just didn't work.
This problem confused me some days, I really need help. Thank you in advance!


texts: The Kepler space telescope points to a single place in the sky, and a quarter of a percent of the sky. How was this field of view chosen? Why did the Kepler team choose to point the telescope there rather than anywhere else?


texts: Is the Alpha Centauri star system moving closer to us?

texts:
The closest star system after our sun is Alpha Centauri, a three star system with Proxima Centauri, a red dwarf, slightly closer to us than the other 2, around 4.3 light years. I tried Wikipedia and other articles that describe Alpha Centauri and there is plenty of information, but no where could a find an answer to said question.

My presumption is that if the distance is measurable, so should the variance in this distance.


texts: If we built one hundred 3m telescopes and point them to a single star what would be the final resolution?

texts: We'll mount all of them in a same location and connect them to each other (like VLT) so they will combined all the results and will produce some staggering images however my question is, would they be able to produce an image close to the resolution of a 300m optical telescope?


texts: Energy conservation in Barnes-Hut algorithm

texts:
I wrote a code that follows the Barnes-Hut algorithm for gravitational dynamics. Everything looks pretty good, except that when I plot the total energy $$E_{tot}\equiv E_k+E_u=const$$ I get this result:

and you can see that although it seems constant for most of the time, it has some increasing transient in the beginning. I couldn't find a bug in the code that could be responsible for this, is there may be other explanation that can make sense of this?


texts: I'd like to photograph NEOWISE from Los Angeles, what is the optimal time?

texts:
I live in Los Angeles and have a camera adapter for my telescope -- nothing fancy just an Astromaster 114. What is the optimal time, in terms of dark sky and bright comet, to take a photo of Comet NEOWISE? I.e., when will it be brightest relative to the surround sky? When will it be largest?
And, lastly, what is the best time tonight to view it?
EDIT: I've spent the last two nights at sunset trying to locate it, but it doesn't really get dark until about an hour after sunset around 8pm. I also spent over an hour on the roof this morning from about 4am til 5:15am. I couldn't see it despite clear skies (or what passes for clear in LA).


texts: Questions about convolving/deconvolving with a PSF

texts:
A point spread function (PSF) has many different uses. Consider for instance the following quote:

  To  extract  the  maximum  information  out  of  an  observation, even the smallest details of the PSF are important. Some examples include: deconvolving the PSF from an observed image to remove the blurring caused by diffraction and reveal fine structure; convolving a model image by the PSF to compare to an observed one;

My question about this is the following:
If we know the PSF of a system, and use this to deconvolve the raw image (purpose 1 from the quote above), why would we convolve a model with the PSF to compare it to an image (purpose 2 from the quote)? That is, can't we just compare the original, unconvolved model, with the deconvolved image? In terms of maximizing the information that seems like the way to go.


texts: How are the speed, distance and time of India's MOM and USA's Maven Mars missions calculated?

texts:
How do the scientists calculate the correct time to launch the Mars mission so that the satellite travel time is less? How they are synchronizing the speed of satellite with respect to earth and mars ?
Refer this link for more understanding:
Animation of MOM and Maven launch


texts:
I am using Python package lightkurve for exoplanets searching by the transit method. When I download light curve of some star and apply periodogram, I find frequency and power of periodic components in the light curve. However, I noticed that multiples (0.5x, 2x, ...) of the original period are displayed too. Here is example for Kepler-6b.
import lightkurve as lk
import numpy as np

light_curve = lk.search_lightcurvefile("Kepler-6", quarter=1).download().PDCSAP_FLUX
light_curve.scatter()

periodogram = light_curve.to_periodogram(method="bls", period=np.arange(0.5, 10, 0.0001))
periodogram.plot()

I could take only strongest period (3.24 d), but what if there are more exoplanets (1.08 d, 9.71 d, ...)? I thought that if I use the light_curve.fold(period) method, I can tell if it is a transit (there is only drop of flux) or not (there are more drops of flux). However, Kepler-20f has also multiple drops of flux after fold light curve (because of other planets?).

How can I tell if it is a planet transit or not?


texts: Average rate of supernova x number of stars

texts: If we would take the average number of supernovae per year and multiply that by the number of stars, could we figure out how many years it will be before all stars are gone?


texts: How is the mass of solar system objects calculated?

texts: When I go to the wikipedia page for any planet in our solar system, the exact mass and volume are reported. How is it possible to do this with them being so far away? I suppose after hundreds of years we can observe them going around the sun, but this doesn't really give any information since we need both the mass and distance from the sun for it to be of any use.


texts: What caused the comet ISON to disintegrate?

texts: So curious to know why comet ISON disintegrated? Well, it went near the Sun but was the Sun the only reason it died? Or is there any other reason for it's fading away?


texts: Why would a black hole have a disk, but not emit x-rays?

texts:
There's a recent paper in Nature about LB-1, a B-class star orbiting a massive black hole.
I don't understand how these two parts of the paper can be reconciled. On page 2, the authors argue that the broad $H\alpha$ line indicates there's a disk in the system, and then that the disk is around the black hole (as opposed to the star or the entire binary).

This supports the $H\alpha$ emission line not coming from a circumbinary disk, but from a disk around the black hole.

However in the last paragraph, they also say that this system doesn't emit X-rays:

Unlike every other known stellar black hole, LB-1 has not been detected in X-ray observations. We searched for X-ray emission from this system with a 10-ks observation with the Chandra X-ray Observatory, placing an upper limit for the X-ray luminosity of $\geq 2 × 10^{31} {\rm erg s}^{−1}$ (see Methods). This upper limit corresponds to about $10^{−9}$ of its Eddington luminosity, and suggests a mass accretion rate ̇$M \leq 10^{-11} M_{\odot} {\rm yr}^{-1}$ for a conversion efficiency of approximately $10^{−4}$ at such low luminosity.

Naively it seems to me that this non-detection is actually an argument that the $H\alpha$ line isn't coming from a disk around the black hole. If there's a disk around a black hole, it's presumably accreting. If it's accreting, the material presumably gets hot and produces X-rays. If it's producing X-rays, Chandra should have detected them.
What am I missing?


texts: How is Cosmic Microwave Background's temperature measured?

texts: How do Cosmic Microwave Background missions/telescopes measure CMB temperatures? I understand that CMB has Planck's spectrum. So I was thinking one strategy could be to measure the spectral radiance corresponding to different frequencies coming from a certain direction and then compared these data points with theoretical Planck's law. Is this technique practically feasible?


texts: Does meteor direction change if you look from a different country?

texts:
So here is the question: Me and a friend of mine from Norway we saw a meteor but que said he saw it in the opposite direction as me and I was looking from Spain, does that make any sense? Or should we see it going the same direction?
Thanks in advance


texts: The laws of physics through out the universe

texts:
How do we know laws of physics are the same throughout the universe? Intuitively I would say they would vary in two natural ways: the constants in the equations may vary or the math in the equations may vary. As a guess they could change over a long time. What is the furthest radius we can prove from earth, with absolute certainty, that the laws of physics do not vary? I am aware this may not be a radius but a more complex shape that cannot be simply described by a radius.

The nearest answer I can think of for a radius is a guess. And that guess is based on the furthest physics experiment we have done from earth. Which I think is an experiment with mirrors on the moon. Therefore if we assume (I don't know if this assumption is totally 100% reasonable) all physics laws hold because this experiment works. Then the radius is to the moon. This doesn't give an concrete answer for the radius, merely an educated guess.


texts: Could anyone tell me what the names of the Greek letters in the screenshot are? Is the first one alpha? They are used in calculating ICRF rotations. Wikipedia uses a different font for their list of characters, so it's hard to tell.


texts: How to locate nebulas visible through AstroMaster 114 telescope without device assistance?

texts:
I will be going to the desert this month and it's my understanding that the night sky will be very dark there. I've got some practice locating the planets but I'd really like to get a look at a nebula or galaxy. I understand from Is the Celestron astromaster 130 EQ Model able to see nebulas and galaxies? that nebulas and galaxies are visible with modest magnification. I'd really appreciate some tips on locating stuff that's cool to look at other than planets and the moon. I've got an AstroMaster 114 (114mm, f/8.77) with a couple of eyepieces and a Barlow lens.

I've searched a bit and these targets seem like a good short list.

Albireo
Orion Nebula, M42
Andromeda Galaxy, M31
Hercules Cluster, M13

I understand that my telescope has an equatorial mount and I do take care to orient it northwards and adjust it to match my latitude, but I've never truly bothered to center it on the north star. I find locating the north star a bit daunting. All the instructions I've encountered say to locate the big dipper (aka the plow) and follow the line of two if its stars. Finding constellations is not my strong suit -- especially with the haze and obstructions here in Los Angeles. I can usually find Orion pretty well, though.

I have an app on my iphone called Planets which I find quite helpful for locating the planets, but I'm not sure if it will operate properly out there where there's no telephone service. It also doesn't provide any tools to identify or locate deep space objects. It's just for planets.

My telescope does have some "setting circles" (little dials demarcated in degrees or hours, see photo) but my understanding of ascension and declination is sketchy at best and I also understand that these are not especially accurate. I'm wondering if they might have some use to get me in the ball park of the objects I'm seeking -- maybe help me at least find the constellation from which I can locate the deep space objects of interest.

So to summarize, I'm hoping for tips on locating the north star and establishing a proper baseline position for my telescope and I'm also hoping for good, basic instructions for locating some interesting, non-planetary objects to view in my scope.


texts: The energy of electromagnetic radiation is related to frequency; higher the frequency, higher the energy level. If electromagnetic waves have lower frequency when they arrive on Earth than originally emitted due to the Doppler effect, where, according to the conservation of energy, does the excess energy go?


texts: How much delta-v does a typical geostationary satellite carry?

texts: I'm wondering - how much delta-v do typical geostationary satellites carry with them once they get into the orbit for things like orbital adjustment or movement to the graveyard orbit?


texts: Shouldn't we not be able to see some black holes?

texts:
Whenever there is a picture of a black hole, (even though they are artist depictions, this is how they are described) most are shown kind of in a flat disk like this.

My question is, why do we see the black hole itself? If it is sucking in material from all directions, then shouldn't the material be kind of covering the black hole, not just spinning around it to be sucked in from only the sides. Especially in the case that a star would be sucked in, then wouldn't all of its light & matter in the accretion disk be encasing the black hole?
In other words, why is the accretion disk flat, and not a sphere surrounding the black hole? So if this were true, we shouldn't be able to see some black holes, only their accretion disks around them.

(unrevised/old post: If a black hole is spherical and has infinite density, then it would be sucking in objects from all directions because the gravity would be the same all around. In other words, the event horizon should be all around it!

I understand everything would orbit the black hole a little before 'falling in', but since the amount of matter and light is so large, shouldn't it encase the black hole, thus making it covered in a ball of light? The event horizon would be a shell so we could not see some black holes, which would mean there could be billions more out there.

As for the other black holes which we have seen/proven, if the information above is true, then maybe they are the result of black holes with poles with low force/gravity/magnetivity/energy. Possibly they could be a disk of some sort or have an unequal distribution of density, either of which could lead to the flat event horizon, maybe even both.

Poles on a black hole would also help account for the quasars shooting out of some. Since the gravitational pull would not be as great from the poles of a black hole, then this would allow the quasars to beam out (otherwise they would have to be going faster than the escape velocity, which is the speed of light).

Maybe the quasars are even the black holes that I was talking about earlier, and this is why you would see no event horizon, only a shell of bright light.

I am no expert in astronomy but if any of these theories sound plausible, please let me know! Just looking for some explanation, I thought of this earlier and couldn't figure it out, I could be completely wrong though. Thanks!)


texts: How much does the orbit of Mars deviate when it is in linear conjunction with Jupiter and Saturn?

texts: Is the gravity of the planets Jupiter and Saturn strong enough to alter the orbit of Mars, or do they just give a small wiggle after which Mars is following his former orbit?


texts:
I am interested in studies and/or publications about planets that wander in space and are not orbiting around a star or its remnants. These dark and frozen worlds should've got ripped away from their respective system by a large passing body (another star? passing black hole) or any other astronomical event (which one?).

I am curious, whether there exist statistical estimates on the number of such planets and the rate at which they are produced. I am also interested in reading about the statistical models that can produce those estimates and encode researchers' beliefs. If there are no such models, why?

A provoking thought: According to Greek, such planets are really fitting their name πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"). It is frightening to imagine such world that drifts deeper and deeper into empty space. Forever.


texts: What are six different methods to measure distances in Astronomy?

texts:
What are six different methods to measure distances in Astronomy in regards of their respective range in terms of astronomical objects?

So far I can name 4: Parallax method, radar ranging, main-sequence fitting, cepheid variables.


texts:
If we take a look at stars more massive than the Sun, their densities vary a lot. UY Scuti is an extremely low-density star that's only 8.5x more massive than the Sun, but is 1000-2000x its size. r136a1, on the other hand, is 256x more massive than the Sun, but is only 30x its size. Neither of them are main-sequence.

The Sun has a lower mass than both of them, yet it's also smaller than both of them.

It doesn't seem like mass makes a big difference. Both reach hydrostatic equilibrium at completely different sizes. r136a1 has a much stronger gravitational pull than UY Scuti, but it should also have much more radiation pressure, right?

So how can we determine the density of a star? Is there a formula?


texts:
So when I look at images taken of Andromeda, I see a lot of individual stars.

Image by André van der Hoeven

It seems to me that the actual individual stars that far away would be too small to resolve, even with a big scope like the Hubble. So, I have to assume that these points of light that I see are either a) stars in our own galaxy, or b) other distant galaxies, like we would see in the Hubble Deep Field:

So I decided to think a little about this ... so I chose what I presume is big, bright star: R136a1. This one is about 30 times the size of the sun, and 8.7 million times as bright. We can barely see this thing with a telescope, and it's 163,000 light years away. Andromeda however, is 2.5 million LY away, over 15 times as far, which would make the light ... what ... 15 squared... 235 times as faint?

I have to imagine that the glow from the galaxy comes from the scattering and reflecting of all the collective starlight inside the galaxy off the dust and gas and whatnot contained therein, and that the actual individual stars would be kind of like individual droplets of water in a cloud. Almost kind of like when you can briefly see the form of a cloud when lightning flashes inside it at night (even though the light really isn't coming from the droplets)

Am I even close to correct in thinking this?

Thanks


texts: How do we measure the age of the universe?

texts:
As mentioned in wiki/Age_of_the_universe,

  The current measurement of the age of the universe is around 13.8 billion years (as of 2015) – 13.799±0.021 billion years

When my friend who is not from science background asks me how do we measure the age of the universe? How should I explain to him?


texts: Why does the Moon appear gray when passing between the Sun and the Earth?

texts:
Shouldn't the Moon appear as bright as a full Moon seen at midnight from Earth?

The photo was taken by DSCOVR at Lagrange point 1.

In the picture, The Moon appears dark gray. Of course the Earth appears bright, reflecting sunlight from clouds and water. The Moon's surface is gray and should reflect less light than the Earth.

It should be irrelevant that we see the far side, since the reflectivity of the Moon's surface should be the same on the far side as the side that faces the Earth.

The midnight full Moon appears much, much brighter as seen from Earth than it does in this picture, despite the fact that the amount of sunlight reflecting from the surface of the Moon is the same in both instances.

I understand the photo was taken with 3 separate exposures of red, blue and green, but this should not affect the brightness.

So why does it appear so dull?


texts:
I don't understand the link between the comoving distance and transverse comoving distance : how can they be equal ?
Here an example of definition that I have found :
Angular Diameter Distance :
The angular diameter distance $D_{A}$ is defined as the ratio of an object's physical transverse size to its angular size (in radians). It is used to convert angular separations in telescope images into proper separations at the source. It is famous for not increasing indefinitely as $z$ -> infinity; it turns over at $z \sim 1$ and thereafter more distant objects actually appear larger in angular size. Angular diameter distance is related to the transverse comoving distance by
$D_{A}=\dfrac{D_{M}}{1+z}(17)$
Some clarifications would be fine.
EDIT 1: @benrg If I have well understood, the transverse comoving distance is simply equal to the comoving distance between object that has emitted at redshift "z" and us that we reveive now this light, isn't it ? If yes, why complicate the things by using the qualified word "transverse" into "transverse comoving distance" ?


texts: How can GAIA's accuracy for position, parallax, and annual proper motion for a given star be the same number?

texts:
Per Wikipedia's Gaia (spacecraft); Objectives:

  Determine the position, parallax, and annual proper motion of 1 billion stars with an accuracy of about 20 microarcseconds (µas) at 15 mag, and 200 µas at 20 mag.

As @RobJeffries painstakingly points out to us in comments below this answer when we talk about parallax as a unit, it is referenced to a baseline of 1 AU. It doesn't refer to the actual cyclic motion seen in the sky due to the orbital motion of whatever is moving around the Sun at its own particular distance.

But the accuracy of both "position" and "annual proper motion" have nothing to do with 1 AU and are measures of positional accuracy of the telescope and how its astrometric data is analyzed.

How can all three of these accuracies simultaneously be 20 µas for 15 magnitude stars?


texts: Going from the second moments of an object to its ellipticities and half-light-radius

texts:
I am studying ellipticity distributions of galaxies and am having trouble moving from the second central moment matrix (covariance matrix) to properties for an object such as its half-light radius.

I already know how to obtain $e1$ and $e2$

They are given by the following:

$e = \frac{Q_{xx} - Q_{yy} + 2iQ_{12}}{N}$

Where $N = N(Q)$ and is just the normalization. The real component is $e1$ and the imaginary component is $e2$

I am wondering how to obtain the half-light radius, as well.

I have seen:

$2r^2 = Q_{xx} + Q_{yy}$ for a circular profile, then one can obtain the HLR from $r^2$ but what about for non-circular profiles? Is the above equation even valid?


texts: What are the "lines" in a constellation or asterism called?

texts:
Is there a technical term for the "lines" in a constellation or asterism? Alternatively, is there an astronomy related coloquialism or any informal term that refers specifically to these lines, and not to the constellation itself?

To illustrate:

In the two examples below, what is the (most?) correct way to describe all of the green lines in the two images.

Argo Navis, an asterism and former constellation.

Source: Wikipedia
The constellation of Draco.

Source: Wikipedia

Further Background/Details:

I was doing some astronomy related browsing and noticed the some of the representations of the same constellation had differences between different sources. The majority were very similar, others had more significant differences.

Version 1

Version 2

Version 3

Version 1 has significantly more lines included in it than Versions 2 and 3.
Version 1 has the line between η Cygni and β Cygni, appears to go via φ Cygni.
Version 3 has the line between η Cygni and β Cygni, goes via a star to the upper right of χ Cygni on the charts.

I want to search for more examples of the different ways the lines in the constellations have been drawn, and all my google fu has failed to produce any useful search terms. Consequently I'm hoping to find there is a term that describes the lines themselves which should be more helpful at finding  material focused on the ways these lines have been drawn.


texts: Does an algorithm exist that takes an image of the night sky and tell you where in the milky way that perspective likely originates?

texts: In Stargate they talk about using the stars on other planets to figure out their location. I know that's fiction, but it made me wonder if such an algorithm (with arguably little usefulness right now) exists.


texts: Orbital decay via gravitational emission in a hypothetical, two-body universe

texts:
Assume we have a universe with nothing in it except for two objects. One of them is a stationary object that is perfectly spherical with the mass of the Moon (without any atmosphere, of course; also, let's call this Body A). The other body is a small satellite similar to the Hubble Space Telescope, orbiting at 100 miles (161 km) above the surface of Body A.
How long would it take for the satellite to crash into Body A via gravitational emission?


texts: A good definition of specific intensity?

texts:
I’m studying specific intensity for a course on astronomical instrumentation. I can’t quite get my head around the proper definition of the term.

One definition I’ve found in various lecture notes / books / powerpoint slides online is that specific intensity is the power per unit frequency INCIDENT upon a unit area INCIDENT from a unit solid angle. An often given example, for instance in the book of Lena, is that if a telescope records a power P from a star, incident from a solid angle S onto a pixel of area A, then the specific intensity of the source would be P/SA per unit frequency.

Now the second definition I keep finding is that the specific intensity is the power per unit frequency, this time passing through a unit area falling into a unit solid angle.

My question is, are these two equivalent? From the 2D geometry equivalent , I know that an incident pencil of rays through a point causes the same pencil to come out again on the other side. But this feels wrong to appeal to, since the physical interpretations feel so different: incident light is very different from emergent light.

If my above interpretation is true, shouldnt there at least be a sign difference between the emergent and incident definitions such that the integral of the specific intensity over all solid angles yields 0 if no absorption/emission takes place?


texts: How to calculate the mean molecular weight of the Sun

texts:
I have a homework question in which I need to estimate a parameter known as $\beta_{P}$ and also the core temperature, both for the Sun.

However, prior to doing this I need to know the mean molecular weight of the Sun.

First question is, how would I go about doing this.

Second question is, can I assume that it is about 70% hydrogen and 30% Helium?


texts: is this really true that speed of light is same as we look from different parts of universe?

texts:
According to the theory of relativity. The speed of light is same for every observer. But how can we know that this applies to more than a local region? What if light travelled to us from a distant region where the speed of light was different or where the relativistic principle did not apply?

Can we design an experiment that refutes both these ideas?


texts: How to use spectral profiles to determine luminosity class?

texts:
I know the luminosity classes are: Ia-0 ( Hypergiants ), Ia ( bright supergiants ), ... , VII ( white dwarf ).

I also have learned that you can use the presence of absorption lines ( ie. use spectral profiles ) to determine which spectral type a star belongs to: presence of helium lines imply O-type, and Hydrogen lines in A-type, getting fainter as the star gets cooler, etc.

I was told that one can use spectral profiles to determine luminosity class ( Ia-0, Ia, Ib, ... ), that the width of the spectral lines indicate the luminosity class: Ia stars have sharp lines, and V stars have broad lines...  How about Ib, II, III, IV, VI, VII?

I mean... how can I distinguish between Ib and II?  Given examples of Ia lines and V lines, you can tell whether an additional line is broader or sharper, but then guess that this additional line is Ib, II, III, or IV?

Is there a more concrete method of determining luminosity class from spectral profiles?


texts: Where should I look at if I had a 25m parabolic radio reflector?

texts:
I am most interested in (exo)planets with atmospheres, see e.g. my question
Requirements to resolve position of Jovian Whistlers up to magnitude of Red Spot with amateur radio equipment?  So if I had a 25m radio dish in my backyard, would I have the chance to observe anything useful about (exo)planetary atmospheres by pointing it at one of the known exoplanets with atmosphere?
Another cool thing to observe would be a fast radio burst. Where should I point my reflector to increase the chance to see one?
References

How big a dish do I need for radio astronomy?
What kind of things I could "see" with an amateur radio telescope?
In German, with some English summaries: Observations at Astropeiler Stockert until 2019, see also Wikipedia about Astropeiler which happens to be an amateur radio observatory with a 25m antenna. I am not associated with them (yet).


texts: We are in Green Lake, WI, 8/12/18, seeing bright orange “planet”/“star” almost due South, perhaps 20 degrees above horizon, what is it?

texts: I have a “sky” app on my ipad and expected it to be Mars, but my app shows Mars just below horizon and closer to East. Oh, I should add that we saw this about 10:15 tonight. Hanging out watching meteor shower, orange object didn’t move much, if at all. I’ve never seen something so orange (or perhaps it is more red).


texts: Can it be assumed that every star in an amateur astrophotograph has been cataloged?

texts:
While I was looking at a wide-angle photograph of the Milky Way I wondered: Is it possible that some of the stars in the image have yet to be identified and/or cataloged?
My answer to this is no, but I would like confirmation or counterexamples for my reasoning:

The usual cited number of stars in the entire night sky that you can see with the naked eye (up to magnitude 6) is from 2500 to 10000 [1]
With a telescope and long-exposure/stacking on a camera, stars of magnitude 10 to 14 can be seen, perhaps even fainter
The Gaia star catalog (although not fully released [2]) contains data for 1.7 billion stars brighter than magnitude 20.7

Assuming that a telescope/camera combination can detect 100 times as many stars than the naked eye can in the entire night sky, if we calculate the proportion of visible to cataloged stars: $$ \frac{n_{visible}}{n_{cataloged}} = \frac{100 \cdot 10e3}{1.7e9} \approx 0.05\% $$
I would consider this a low enough value to conclude that every star above a certain magnitude has been detected and cataloged. Even seeing a thousand times more stars than the naked eye would give you 0.5% - a negligible proportion of the star catalog.
Is this justifiable? Can these assumptions be challenged?
[1] - https://earthsky.org/astronomy-essentials/how-many-stars-could-you-see-on-a-clear-moonless-night
[2] - https://irsa.ipac.caltech.edu/Missions/gaia.html


texts: How exactly can the hypothetical conformal invariance of the CMB spectrum be established by analyzing tensor modes?

texts:
In the introduction of this paper at the top of p11, it is mentioned that a hypothetical enhancement of the scale invariance of the CMB spectrum to conformal invariance could potentially be established by analyzing the higher n-point functions of the tensor modes.

First of all, why are the tensor modes needed for this? Does conformal invariance not appear in the scalar modes?
How does this work exactly (mathematically), how does conformal invariance (not just scale invariance...!) manifest itself in the n-point functions of the tensor modes?

Has this already been looked at or will it be done for the tensor modes in the BICEP2 data for example?